Field of the Invention
The present invention relates in general to phototherapy, and more particularly, to novel apparatuses and methods for phototherapy of cardiac tissue.
Description of the Related Art
Myocardial ischemia refers to the condition of oxygen deprivation in heart muscle (“myocardium”) that is produced by some imbalance in the myocardial oxygen supply-demand relationship. Myocardial infarction (“MI”), also known as “heart attack”, refers to the death of cells in an area of heart muscle as a result of oxygen deprivation due to obstruction of the blood supply, typically due to occlusion of one or more coronary arteries or branches. Occlusion usually stems from clots that form upon the sudden rupture of an atheromatous plaque through the sublayers of a blood vessel, or when the narrow, roughened inner lining of a sclerosed artery leads to complete thrombosis. Approximately 1.5 million myocardial infarctions (MIs) occur annually, and nearly 500,000 deaths result from ischemic heart disease. The United States alone loses billions of dollars annually to medical care and lost productivity due to cardiovascular disease including myocardial infarction.
Treatment after MI depends on the extent to which the cells have been deprived of oxygen. Complete oxygen deprivation produces a zone of infarction in which cells die and the tissue becomes necrotic, with irretrievable loss of function. However, immediately surrounding the area of infarction is a less seriously damaged region of tissue, the zone of ischemia, in which cells have not been irretrievably damaged by complete lack of oxygen but instead are merely weakened and at risk of dying. If adequate collateral circulation develops, the extended zone may regain function within 2 to 3 weeks. The zone of infarction and the zone of ischemia, are both identifiable using standard diagnostic techniques such as electrocardiography, echocardiography and radionuclide testing.
Therapeutic strategies in treating MI are directed at reducing the final extent of the infarcted region by preserving viable tissue and if possible retrieving surviving but at-risk cells. Known treatment methods for myocardial infarction include surgical interventions and pharmacologic treatments. A combination of therapeutic approaches is sometimes advisable. Selection of the appropriate therapy depends on a number of factors, including the degree of coronary artery occlusion, the extent of existing damage if any, and fitness of the patient for surgery. Surgical interventions include coronary artery bypass surgery and percutaneous coronary procedures such as angioplasty, artherectomy and endarterectomy. Pharmacologic agents for treating MI include inhibitors of angiotensin converting enzyme (ACE) such as captopril, quinapril and ramipril, thrombolytic agents including aspirin, streptokinase, t-PA and anistreplase, β-adrenergic anatagonists, Ca++ channel blockers, and organic nitrates such as nitroglycerin. However, surgical interventions are invasive and can increase the risk of stroke, and pharmacologic agents carry the risk of eliciting serious adverse side effects and immune responses.
High energy laser radiation is now well accepted as a surgical tool for cutting, cauterizing, and ablating biological tissue. High energy lasers are now routinely used for vaporizing superficial skin lesions and, and for making deep cuts. Examples of such procedures include transmyocardial laser revascularization (TMLR) and percutaneous transmyocardial laser revascularization (PTMR). In TMLR, a laser is inserted through a chest incision and used to drill approximately 15-30 transmural channels from the epicardial to the endocardial surfaces through the left ventricular myocardium in an attempt to improve local perfusion to ichemic myocardial territories not being reached by diseased arteries. In PTMR, the laser is introduced via a catheter. Other examples include laser ablation or cauterization of cardiac tissue to stop atrial fibrillation.
For a laser to be suitable for use as a surgical laser, it must provide laser energy at a power sufficient to heat tissue to temperatures over 50° C. Power outputs for surgical lasers vary from 1-5 W for vaporizing superficial tissue, to about 100 W for deep cutting.
In contrast, low level laser therapy involves therapeutic administration of laser energy to a patient at vastly lower power outputs than those used in high energy laser applications, resulting in desirable biostimulatory effects while leaving tissue undamaged. In rat models of myocardial infarction and ischemia-reperfusion injury, low energy laser irradiation reduces infarct size and left ventricular dilation, and enhances angiogenesis in the myocardium. (See, e.g., Yaakobi et al., J. Appl. Physiol., Vol. 90, pp. 2411-19 (2001)).
Against the background, a high level of interest remains in finding new and improved therapeutic methods for the treatment of myocardial infarction. In particular, a need remains for relatively inexpensive and non-invasive approaches to treating myocardial infarction that also avoid the limitations of drug therapy.